1. Field of the Invention
The present invention relates to a pneumatic tire having an improved tread pattern to reduce pattern noise. More particularly, the present invention relates to an improvement in a variable pitching method.
2. Description of the Related Art
In general, tires,are provided in the tread portion with circumferentially spaced tread elements constituting a tread pattern such as blocks for good road grip. During running, such tread elements contact with the road surface one after another, and noisy, sound called pattern noise is generated.
In order to reduce such pattern noise, the so called variable pitching methods have been proposed. In those methods, pattern noise is modulated into a wide frequency range so that the pattern noise turns to the so called white noise. For example, in laid-open Japanese patent application Nos. JP-A-8-108711, JP-A-8-113012 and JP-A-8-113013, such a method is disclosed.
In the variable pitching methods, by increasing the number of different pitches, it becomes easy to turn pattern noise to white noise. However, when the pitch number is large, the rigidity of the tread elements is liable to become uneven and uneven wear increases.
It is therefore, an object of the present invention to provide a pneumatic tire, in which the occurrence of uneven wear can be controlled in spite of a relatively large number of different pitches used therein to improve pattern noise.
According to one aspect of the present invention, a pneumatic tire comprises
a tread portion provided with a series of design-cycles repeating in the circumference of the tire at variable pitches, wherein the number (NP) of different kinds of pitches is at least 10,
said variable pitches repeating increase and decrease alternately in one circumferential direction of the tire to define start design-cycles from which the pitches starts to increase,
said series of design-cycles composed of a series of units, each unit defined as a group of design-cycles starting from one of the start design-cycles inclusive to the next start-cycle exclusive,
the total number of the units being in the range of from 4 to 20,
the units consists of
at least one first-unit the number of which is 20 to 80% of said total number,
at least one second-unit the number of which is 20 to 80% of said total number, and optionally
a third-unit the number of which is 0 to 60% of said total number,
wherein the first unit is defined as including
at least one pitch which is not less than a standard long pitch (PLa) and
at least one pitch which is not more than a standard short pitch (PSa),
the second unit is defined as including
at least one pitch within one of the following two ranges,
a range of not less than the standard long pitch (PLa) and
a range of not more than the standard short pitch ( PSa), but no pitch within the other range,
the third unit is defined as all the pitches therein which are more than the standard short pitch (PSa) and less than the standard long pitch (PLa),
the standard long pitch (PLa) is the maximum pitch (PL) minus a length of 0.2 times a maximum pitch difference (MPD),
the standard short pitch (PSa) is the minimum pitch (PS) plus a length of 0.2. times the maximum pitch difference (MPD) and
the maximum pitch difference (MPD) is the maximum pitch (PL) minus the minimum pitch (PS).
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.
Embodiments of the present invention will now be described in detail in conjunction with the accompanying drawing, in which: